Magnetoresistors are devices whose electrical resistance varies with a magnetic field applied to them. They are useful as magnetic field sensors; for example, as moving element detectors in mechanical position sensing applications. FIGS. 1a and 1b of the drawings herein show magnetoresistors at 10 and 11 used in a representative position sensing application. The magnetoresistors 10 and 11 are mounted on top of magnetically conductive support elements such as the ferrous pole members shown at 13 and 14 in the drawings and are used, for example, to sense the passage of ferrous turbine blades 12 and 17 of an aircraft engine. The magnetoresistors sense turbine blade position by responding to a change in local magnetic field strength, which results in a change in the electrical resistance of the intrinsic material of the magnetoresistor.
In FIG. 1a, the quantity of magnetic flux flowing through the magnetoresistor 10, as a result the magnetic flux 15 coupling through the air to the engine rotor, is at a relatively high value with the illustrated close alignment of the blade 12 with the magnetoresistor 10. FIG. 1b shows a decrease in the magnetic flux for the misaligned condition. Other position sensing applications for magnetoresistor magnetic field sensors include automotive engine applications, such as measuring revolution rates and timing events such as fuel injection or fuel ignition, for commutation in brushless electric motors, for naturally occurring magnetic field sensing such as in navigation and terrain responsive guidance systems, in sensors for satellites, and any number of applications where precision motion sensing is required. The noncontact method of magnetic field sensing as accomplished with a magnetoresistor provides longer reliability and longer device service life than is available from contacting arrangements, and in many instances, is more convenient than either voltage generating sensing and optical sensing.
FIG. 2 is a graph showing electrical resistance on the y-axis, 20, as a function of magnetic field intensity, H, on the x-axis, 21, for a typical magnetoresistor element. So long as the magnetoresistor element is located in air surroundings of unity permeability, the magnetic field intensity H and magnetic flux density are equal. As is well known in the electrical art, however, this relationship is often not true in a ferrous magnetic circuit where such effects as saturation of a magnetic member can occur. Curve 22 in FIG. 2 represents the relationship that an increase in electric field intensity produces a related increase in the electrical resistance of the intrinsic material of the magnetoresistor. More specifically, the maximum resistance in the intrinsic material is obtained at a maximum magnetic field intensity and a minimum resistance is obtained at a minimum magnetic field intensity. This relationship is represented mathematically by the following equation: EQU .DELTA..rho./.rho.=g.mu..sup.2 (.DELTA..beta.).sup.2 Eq. 1
where .DELTA..rho. represents material electrical resistance, g represents a shape factor for the resistor, .mu. represents mobility and .beta. indicates flux density in the magnetic field. In words, the mathematical relationship provides that a change in intrinsic material resistance is proportional to the square of the change in the magnetic field multiplied by the square of the mobility of the intrinsic material.
A change in magnetoresistor device electrical resistance is also proportional to a change in intrinsic material resistance multiplied by the device length and divided by the product of the device thickness and width as represented by the following mathematical equation: EQU .DELTA.R=(.DELTA..rho.L)/(tw) Eq. 2
where L, t and w are sensor device length, thickness and width, respectively, and .DELTA.R is change in sensor device electrical resistance. This is further illustrated in FIG. 3 which is a schematic of a magnetoresistive sensor where active layer thickness is shown at 32, active layer length is shown at 30 and magnetic field density is shown at 34.
The sensor signal--the desired output of the magnetoresistive sensor--is the voltage which is proportional to the change in device resistance multiplied by the device current shown at 33 in FIG. 3, represented mathematically by the equation: EQU .DELTA.V=I.DELTA.R Eq. 3
where .DELTA.V is the change in voltage, I is the current which is presumed constant and .DELTA.R is the change in device resistance.
Semiconductors have typically been used to create magnetic field sensors such as magnetoresistors. Magnetoresistors are believed to be best formed from high electron mobility semiconductors; that is, by depositing high electron mobility carrier films on a mechanical substrate--an arrangement believed to achieve the highest magnetic sensitivity. Considering Eq. 1, it is apparent that an increase in material electron mobility significantly increases material resistance by a power of two. Considering Eq. 2, it is apparent that a reduction in active layer thickness also produces an increase in device resistance. Therefore, an increase in material electron mobility combined with a reduction in active layer thickness produces a significant increase in device resistance. As shown in Eq. 3, an increase in device resistance allows a reduction in current while still obtaining the same voltage or sensor signal.
In view of these relationships, prior work focused on methods of depositing active films of decreasing thickness onto a sensor substrate. However, due to lack of a suitable substrate--that is, lattice constant and crystal structure matching those of the active film layer--high dislocation densities result and the thin films do not exhibit the desired high material electron mobility which achieves the highest magnetic sensitivity. The density of dislocations is expressed as the dislocation line unit per unit volume, cm/cm.sup.3 or cm.sup.-2. The dislocation line units are imperfections in the crystalline lattice spacing and commonly result to accommodate strain induced due to lattice mismatched materials. The thinning process in the method of the prior art induces defects in the device which reduce the electron mobility of the active layer below desirable limits. Accordingly, to avoid such defects, common magnetoresistors are made in the order of microns in thickness. A conventional magnetoresistor has an electron mobility in excess of 25,000 centimeters squared per volt second and a thickness of a few microns (typically 1-3). At these dimensions, the resistance of a typical device is only a few ohms. As a result, a device layout with hundreds of individual resistors connected in series allows for a total device resistance in the kilohm range. At this total device resistance, a current in the milliampere range is still needed in order to create a sufficient voltage change for measurement purposes. This amount of current causes detrimental device heating effects and limits the ability to operate these sensors using batteries.
The invention provides a method for making magnetoresistors that have a dislocation density reduced by at least five orders of magnitude. Magnetoresistors produced by the method of the invention have improved performance because an ultrathin active film is deposited on a compliant substrate. Depositing an ultrathin active film layer on a compliant substrate results in a higher device resistance with lower current requirements for device operation. A reduced sensor current for operation of magnetoresistors reduces problems due to device heating and allows for battery operation of such devices.